This invention relates generally to pulse detonation engines, and more particularly to the fluid dynamics of such engines. More particularly still, the invention relates to the valving of fluids employed in pulse detonation engines.
Pulse detonation engines (PDE) represent an energy conversion device that has existed for some time, but which have recently received increased attention. Such engines generally combust fuel and an oxidant in a chamber, or combustor, to provide a component of thrust or force in an intended direction. The combustion occurs in the manner of discrete, i.e. pulsed, detonations. The present invention is concerned with configurations that employ an open-ended chamber, such as a rocket nozzle, that may be employed for a component of thrust. A principal application for such engines is as a thrust source to propel an aerospace vehicle in the atmosphere or the vacuum of space. In such instance, the PDE is also a rocket engine.
The development of PDE""s requires the ability to quickly fill an open-ended chamber with a detonatable mixture while purging the exiting exhaust gases with minimal mixing. For pulse detonation rocket engine applications where the open end of the chamber is likely to be exposed to a vacuum, the fresh charge of detonatable mixture must be contained in the chamber for several milliseconds before the detonation is initiated. Further, the mixture must be pressurized to levels on the order of 50-1000 psi with the chamber open to vacuum in order to generate thrust competitive with conventional rocket engines. Moreover, the process must be done at rates approaching or exceeding 100 Hz.
Two alternative concepts that may be used to control chamber pressure are mechanical valves and fixed throats, but each has significant limitations. The complexity and weight of a mechanical valve, combined with durability and sealing requirements in the hot exhaust flow, suggest that it would be difficult and/or impractical to implement in this application and environment. A fixed throat near the chamber exit would restrict the flow, allowing the chamber to be pressurized with high propellant flow rates while some propellant is lost through the exit. This arrangement suffers from the loss of efficiency due to propellant leakage during the fill process and from thrust reduction due to the reduced exit area of the throat.
A further discussion of the physics and operation of PDE""s is contained in several U.S. Patents by T. R. Bussing, including U.S. Pat. Nos. 5,513,489; 5,353,588; and 5,345,758. These patents discuss the use of a rotary valve, but only to control the admission of propellant and oxidant to each of multiple combustor chambers rather than to also control the pressure developed in the chamber. They rely upon an approach that uses multiple chambers each feeding into a common, restricted throat.
Accordingly, it is an object of the invention to provide an arrangement that is durable, relatively efficient, and simple, for controlling the pressure of fluids, such as propellants, in the chamber of a pulse detonation engine.
It is a further object to provide a pulse detonation engine in which the timing of detonation is optimized or tuned in accordance with the present invention.
It is a still further object to provide improved fluid injection mechanism for use with the pulse detonation engine in accordance with the invention.
A pulse detonation engine (PDE) is provided with an aerodynamic valve (aerovalve) for controlling the pressure of injected fluids, typically gases, but also including liquids, such as fuel or fuel and oxidizer, referred to as propellants, in an open-ended detonation chamber. The PDE includes a detonation chamber closed at a thrust wall end and open at the opposite, exhaust, end. A fluid injection mechanism injects pressurized propellant fluid into and directed toward the thrust wall end of, the detonation chamber. The propellant fluid is injected in a pulsed manner by the injection mechanism, and with sufficient pressure and velocity and such direction as to effectively restrict or prevent rearward flow of the injected fluid, thus forming a closed aerovalve. Moreover, the injected propellant fluid establishes a shock wave which, when moving rearward in the detonation chamber toward the exhaust end after reflection by the thrust wall end, serves in combination with the closed aerovalve to increase, or at least sufficiently conserve, the pressure of the fluid in the chamber, such that it is a pressure preferably greater than, or at least nearly as great as, that at which it entered. At the appropriate instant, the injected and pressurized propellant is detonated, as by an ignition device, to rapidly combust the injected propellants. Because the exhaust end of the detonation chamber is not mechanically constricted, as by a mechanical valve, and the aerovalve is open because injection has ceased, the resulting combustion products readily exit to produce thrust. Such configuration avoids the concerns of a mechanical valve structure for controlling exhaust from the detonation chamber and does not require a mechanically-constricted throat structure which would otherwise impede exhaust flow.
The PDE is preferably tuned to optimize the specific impulse or thrust. This is accomplished by coupling the timing of the fluid propellant injection and the subsequent ignition to maximize performance. More specifically, the mass average chamber temperature and pressure are at a maximum as the reflected shock wave nears the injection mechanism. The ignition is timed and positioned such that the resulting detonation wave, which moves at a velocity greater than the reflected shock wave, arrives at the injection mechanism at the same instant as the reflected shock wave.
The propellant injection mechanism is a high frequency pulse valve which delivers and injects pressurized pulses of propellant, typically fuel and oxidizer, to the detonation chamber of the PDE. Various arrangements may be used for providing such pressurized pulses of propellant. However, in the propellant injection mechanism of the preferred embodiment, each propellant component is supplied, at pressure, to a respective slotted disk type of valve. The slotted disk valve includes at least one rotating disk having a plurality of slots, and a complementary slotted disk, typically stationary, such that the slots of the two disks port (open) and unport (close) in rapid succession. The propellant pulses are delivered from the disk valves to the detonation chamber via injection ducts which are positioned and directed to provide the aerovalve of the invention.
One embodiment of the propellant injection mechanism comprises one or more injection valves offset from the axis of the detonation chamber and in which the rotating disk is driven by a spring-biased drive member at the axis of the disk, but offset from the detonation chamber axis.
Another embodiment of the propellant injection mechanism comprises a pair of annular injection valves that encircle the detonation chamber and in which annular fixed and rotating slotted disks are disposed coaxially with the detonation chamber to provide the pulsed injections.